1. Field of the Invention
The invention relates to the field of radiation detection probes and in particular to a beta or positron probe used during surgery to intraoperatively detect tissue labelled with positron or electron emitting isotopes.
2. Description of the Prior Art
The goal of oncological surgery is to completely remove neoplastic cells while sparing adjacent normal cells. This surgery is often either impossible or fails because the tumor has infiltrated the tissue to form small fingers which cannot be detected by visual examination or numerous small tumors which are dispersed throughout a large area of tissue. Failure to remove such diseased tissue often only leads to a continuation or recurrence of the cancer.
Radiolabelled monoclonal antibodies and other radiopharmaceuticals have been developed which preferentially bind to cancer cells. Detection and removal of these radioactively tagged cancer cells increases the success rate of such surgeries and can be realized if the tagged cells can be accurately located.
Prior art attempts to develop intraoperative probes for tumor location are limited in their usefulness because of their high sensitivity to gamma radiation. The range of gamma radiation in body tissue is large and therefore accumulations of the radioactive marker in a distant organ can create a high background gamma radiation making the detection of nearby tagged tissues difficult or impossible.
One prior art attempt to distinguish marked near tissues from marked distant tissues is shown by Hickernell, "Dual-Detector Probe for Surgical Tumor Staging", The Journal of Nuclear Medicine, Vol. 29, No. 6, June 1988, pages 1101-1106. Hickernell shows a dual probe which simultaneously monitors gamma counts from a possible tumor site along with counts from adjacent normal tissue using two concentric, collimated scintillation detectors. A comparison of the counts between the central axial and concentric detectors is used to distinguish a small tumor, which is directly in front of the probe, from tissue sources generating background activity. Hickernell has a first scintillation detector axially disposed in the probe with a second scintillation detector coaxially disposed about the first detector. Two lead collimators extend forwardly of the scintillation detectors. Like the detectors, the lead collimators are coaxially concentric with each other so that the two collimators divide the radiation incident of the end of the probe between the center axial detector and circumferential coaxial detector.
The theory is that a distant source of radiation will uniformly radiate both the center and circumferential detectors and that a close source of radiation, if placed in line with and laterally extending no more than the diameter of the center collimator will provide radiation predominately to the center detector. The difference in radiation received between the center detector and the circumferential detector is determined to identify a radiolabelled small tumor immediately in front of the center collimator of the probe.
The limitation of Hickernell's device is that if the near tissue source of the radiation is larger than the aperture area of the central collimator of the probe, it will provide radiation to both the circumferential detector as well as the center detector. The probe will therefore be blind to near tumors larger than the size of the central collimator aperture. Furthermore, it should be noted that Hickernell's device is a gamma detector and has no ability whatsoever to preferentially detect beta particles. Hickernell's device is sensitive to any background gamma radiation noise that my exist.
Heretofore, beta radiation has not been significantly used in medical applications since the range of beta particles within tissue is relatively short, typically from a micron to a few millimeters depending upon the energy of the particle. In particular, positron emitters have rarely been used since the half life of biologically useful positron emitters is fairly short, typically between an order of a few seconds to a 100 minutes. However, the recent common establishment of cyclotron facilities at advanced medical centers throughout the world has provided a practical opportunity for the production of such short half-life beta markers. The positron emitter is manufactured at the cyclotron and used on site or at a location which is within one to two hour's shipment time of the cyclotron site.
Positron emitters have the advantage that many of the natural organic elements which occur in the body in abundance have emitting isotopes analogs, have high energies, short half-lives which reduce post operative tissue handling and disposal radiation hazards.
Phosphorous 32, for example, a high energy beta emitter, has been used in the prior art to mark brain tumor tissue and has been successfully detected with a semiconductor detector. Phosphorous 32 is advantageous in that it generates high energy beta radiation with no gamma radiation. The problems dealt with in Hickernell therefore do not occur since there is no background gamma radiation. However, the applicability of phosphorous 32 as a marker is limited because it does not localize well in some tissues such as gliomas, it has a long half-life and is toxic to bone marrow.
Therefore, it is anticipated that if beta radiation is to be used as a tissue marker, some type of apparatus and methodology must be devised to detect the beta radiation notwithstanding the existence of background gamma radiation generated by the normally used types of radioactive markers.